Current immunosuppressive strategies have decreased the incidence of acute cellular rejection of renal and cardiac allografts. The incidence of acute humoral rejection (AHR) in renal transplant patients is increasing and is particularly difficult to treat. The occurrence of AHR is associated with donor-specific antibody binding to the graft endothelium followed by neutrophil and macrophage infiltration into the graft with subsequent interstitial fibrin deposition. Antibody mediated mechanisms leading to graft injury and loss remain poorly understood. Investigation into these mechanisms is hampered by the lack of good animal models to study the development of allograft injury as the donor-specific antibody response is initiated and increases and to develop strategy is to inhibit this antibody-mediated injury. For the most part, models studying acute humoral rejection use either transfer of graft-reactive antibodies or sensitization of recipients with donor cells which also primes donor-reactive T cell populations. During studies investigating the role of CCR5 in acute cellular rejection of cardiac allografts, we recently observed that the titers of donor-specific antibody in CCR5-deficient recipients were almost 20-fold higher than in wild-type recipients and the rejection of the allografts in the CCR5-deficient recipients was characteristic of acute humoral rejection. We have now extended these studies to the rejection of MHC-mismatched renal allografts. In wild-type C57BL/6 recipients, A/J renal allografts experience tubulitis with infiltration of CDS T cells but this resolves and the allograft is able to support the life of the recipient. In CCR5-deficient animals, the renal allografts are rejected between days 10 and 20 with heavy deposition of C3d, peritubular edema and neutrophil infiltration. At the time of rejection, the titers of donor-specific antibody in the CCR5-deficient recipients are more than 25-fold higher than those observed in wild-type recipients. These results have led us to propose the hypothesis that renal allografts experience acute humoral rejection in CCR5-deficient recipients. This hypothesis will be tested in three specific aims. In Specific Aim 1 we will directly test the role of antibody in mediating rejection of the renal allografts. In Specific Aim 2 we will test the induction of proinflammatory cytokines and their roles in mediating the rejection. In the final specific aim we will use a B cell depletion strategy to test the effect of limiting donor-specific antibody interaction with the renal allograft on immediate and long term graft injury.